Given how the game ended, and how long it been since it happened, it's just too late, and a good reason of why this kind of DLC sometimes just doesn't work.

I'd be interested in playing the Omega mission, has it been avaliable at launch but now? Meh. And I'm sure no matter what I do in Omega there will be no change in the end-game. I bet Aria will even keep hanging on the same bar spot after you finish it all.

Going back to Mass Effect now is like going back to finish a meal you let in the fridge for 1 month. It may still be edible but... eeeew.